In Gen Z and broader Internet language, "sus" is a shortened form of "suspicious" or "suspect," meaning something or someone seems questionable, untrustworthy, or not right.
Gamitin natin ng bongga (with references to High Street's Governor William Acosta played by Mon Confiado):
Gov. William Acosta is the Proprietor of Northford High School. An abusive and sharp-tongued politician, he rejected a request for an investigation into the tragic death of a student, and ruled it out as suicide.
This was totally "sus" of him. Later, it was revealed that he was actually the one responsible for the murder.
Related Links:
Mon Confiado bids goodbye to 'High Street':